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5.1.5.3 <br />For confined aquifers, the Minnesota DNR has established a two -tiered aquifer protection <br />threshold system to ensure the long-term viability of the pumped aquifer and to prevent <br />exceedance of the aquifer safe yield as defined by MN Rule 6115.0630 Definitions Subps.15 and <br />16. These thresholds allow for appropriation from the aquifer but establishes minimum water level <br />elevations to be maintained as a safeguard to protect the structural integrity of the aquifer itself. <br />Threshold elevations are set in observation wells completed in the source aquifer and not <br />pumped wells. <br />• The first threshold is set at an elevation that is 50% of the pre -pumping available head <br />above the top of the aquifer. If water levels drop to the 50% threshold, pumping will need <br />to be evaluated and a possible reduction in rate and volume may be required. <br />• The second is a water level elevation associated with 25% of the pre -pumping available <br />head above the aquifer. At the 25% threshold, pumping would need to cease to prevent <br />exceeding the safe yield for the artesian aquifer. <br />• If more than one aquifer is impacted by pumping, then thresholds are set similarly in the <br />other aquifers. <br />The table below depicts the static water level and the approximate available head above top of <br />aquifer in the City of Ramsey's current municipal wells taking account the Minnesota DNR 50% <br />and 25% thresholds. <br />Table 11— Drawdown in Ramsey Wells <br />Well <br />N <br />Unique <br />Well No <br />Tota <br />epth <br />(ft) <br />Casin <br />Depth <br />(ft) <br />tatic Water <br />Level Depth <br />(ft) <br />Approximate <br />Drawdown to <br />Top of <br />Aquifer (ft) <br />Approximate <br />Drawdown to <br />50% <br />hreshold (ft) <br />Approximate <br />Drawdown to <br />25% <br />threshold (ft) <br />1 <br />161441 <br />323 <br />243 <br />9.5 <br />233.5 <br />116.75 <br />175.1 <br />2 <br />416183 <br />320 <br />240 <br />9.5 <br />230.5 <br />115.25 <br />172.8 <br />3 <br />580306 <br />345 <br />222 <br />26 <br />196 <br />98 <br />147 <br />4 <br />580313 <br />321 <br />191 <br />18 <br />173 <br />86.5 <br />129.7 <br />5 <br />593672 <br />316 <br />215 <br />25 <br />190 <br />95 <br />142.5 <br />6 <br />706840 <br />390 <br />282 <br />37 <br />245 <br />122.5 <br />183.7 <br />7 <br />743832 <br />332 <br />216 <br />25 <br />191 <br />95.5 <br />143.2 <br />8 <br />743833 <br />354 <br />245 <br />15 <br />230 <br />115 <br />172.5 <br />These MnDNR threshold values are approximate as an observation well has to be established by <br />the MnDNR for baseline water elevations; however, the table above provides an estimate for <br />which the city should manage water levels. <br />Well Specific rapacity <br />Well specific capacity is the rate of pumping per unit of drawdown expected. This is generally <br />expressed as gallons per minute (GPM) per foot of drawdown. All of the City of Ramsey wills <br />experience a sharp initial displacement of water as the wells are turned on (as seen in Well 5 <br />drawdown examples above). For the purpose of this report, this initial displacement was not <br />considered in the calculations for Well Specific Capacity in order to give a more accurate <br />depiction of how the aquifer will respond once stabilized to various pumping rates. <br />FEASIBILITY STUDY RAMSY 154354 <br />Page 20 <br />